1. Field of the Invention
The present invention relates to a motor drive device and, more particularly, to a motor drive device for a brushless multi-phase DC motor.
2. Description of the Related Art
FIG. 1(a) is a circuit block diagram showing a conventional brushless motor drive device. Referring to FIG. 1(a), a motor M is a three-phase DC brushless motor having three phase coils U, V, and W. Three Hall sensing elements 11u, 11v, and 11w are arranged around the motor M for generating three Hall sensing signals HU, HV, and HW in response to variations in the magnetic field of the motor M. Based on the Hall sensing signals HU, HV, and HW, a signal synthesizing circuit 12 generates three sinusoidal drive signals SU, SV, and SW. Subsequently, the sinusoidal drive signals SU, SV, and SW are input to a pulse width modulation (PWM) comparing circuit 13 for being individually compared with respect to a high-frequency triangular reference signal T generated by an oscillating circuit 14. Based on the comparison of the sinusoidal drive signals SU, SV, and SW individually with the high-frequency triangular reference signal T, the PWM comparing circuit 13 generates three pulse signals PU, PV, and PW to be supplied to three pre-drivers N1, N2, and N3, respectively.
FIG. 1(b) is a waveform diagram showing an operation of a conventional brushless motor drive device. For the sake of simplicity, only is illustrated in FIG. 1(b) the operational waveforms associated with the coil U of the motor M since each of the phase coils U, V, and W of the motor M is operated with similar waveforms. Referring to FIG. 1(b), the pulse signal PU is generated from the comparison of the sinusoidal drive signal SU and the high-frequency triangular reference signal T through using the PWM comparing circuit 13. More specifically, the HIGH level of the pulse signal PU corresponds to an interval of time when the sinusoidal drive signal SU goes higher than the high-frequency triangular reference signal T and the LOW level of the pulse signal PU corresponds to an interval of time when the sinusoidal drive signal SU goes lower than the high-frequency triangular reference signal T. In response to the pulse signal PU, the pre-driver N1 generates the switching signals UH and UL for controlling the switches S1 and S2, respectively.
A three-phase switching circuit 15 has a pair of switches S1 and S2, a pair of switches S3 and S4, and a pair of switches S5 and S6, each pair being controlled by one corresponding pair of the switching signals UH and UL, VH and VL, and WH and WL. A motor drive current Im is allowed to flow from a drive voltage source Vdd to the coil U when the switch S1 becomes short-circuited and to flow from the coil U to a ground potential when the switch S2 becomes short-circuited. The motor drive current Im is allowed to flow from the drive voltage source Vdd to the coil V when the switch S3 becomes short-circuited and to flow from the coil V to the ground potential when the switch S4 becomes short-circuited. The motor drive current Im is allowed to flow from the drive voltage source Vdd to the coil W when the switch S5 becomes short-circuited and to flow from the coil W to the ground potential when the switch S6 becomes short-circuited.
For detecting the motor drive current Im, a resistor Rs is series-connected between the common connecting point of the switches S2, S4, and S6 and the ground potential. A voltage difference caused by the motor drive current Im flowing through the resistor Rs is supplied as a negative feedback to an inverting input terminal of an error amplifier EA. The error amplifier EA compares the voltage difference representative of the motor drive current Im with a current command signal Icom for generating a current error signal Ierr. Subsequently, the signal synthesizing circuit 12 adjusts the amplitudes of the sinusoidal drive signals SU, SV, and SW in accordance with the current error signal Ierr.
FIG. 1(c) is a circuit block diagram showing a conventional signal synthesizing circuit 12. Referring to FIG. 1(c), the signal synthesizing circuit 12 has a position detecting circuit 20, a phase shifting circuit 21, and three multiplying circuits 22u, 22v, and 22w. On the basis of the Hall sensing signals HU, HV, and HW, the position detecting circuit 20 determines positions of a rotor (not shown) in the motor M and then generates three position signals 23u, 23v, and 23w. The phase shifting circuit 21 makes the phase of each of the position signals 23u, 23v, and 23w shifted by 30 degrees so as to form three sinusoidal control signals 24u, 24v, and 24w, respectively. Finally, through the multiplying circuits 22u, 22v, and 22w, the sinusoidal control signals 24u, 24v, and 24w are multiplied by the current error signal Ierr so as to become the sinusoidal drive signals SU, SV, and SW. Therefore, the amplitudes of the sinusoidal drive signals SU, SV, and SW are effectively adjusted in accordance with the current error signal Ierr.
However, in the conventional signal synthesizing circuit 12, the amplitudes of the sinusoidal drive signals SU, SV, and SW are also subjected to the influence of the Hall sensing signals HU, HV, and HW. More specifically, during the procedure where the position detecting circuit 20 together with the phase shifting circuit 21 generate the sinusoidal control signals 24u, 24v, and 24w on the basis of the Hall sensing signals HU, HV, and HW, the amplitudes of the Hall sensing signals HU, HV, and HW are preserved and passing on, such that each of the sinusoidal control signals 24u, 24v, and 24w has an amplitude in proportion to the amplitude of the corresponding one of the Hall sensing signals HU, HV, and HW. Typically, the Hall sensing signals HU, HV, and HW generated from the Hall sensing elements 11u, 11v, and 11w have amplitudes that are influenced by the size and parameters of the Hall sensing elements and the surrounding temperature during operation. As a consequence, the amplitudes of the sinusoidal control signals 24u, 24v, and 24w are changed along with the variations of the amplitudes of the Hall sensing signals HU, HV, and HW, even if the current error signal Ierr remains constant. Since the amplitudes of the sinusoidal drive signals SU, SV, and SW have effects on determining the duty ratios of the pulse signals PU, PV, and PW generated from the PWM comparing circuit 13, the conventional signal synthesizing circuit 12 renders the operation of the motor M subjected to the variations of the Hall sensing signals HU, HV, and HW, which is a disadvantage with respect to motor's stability of rotation.